In the art of tissue making and papermaking in general, many additives have been proposed for specific purposes, such as increasing wet strength, improving softness, or control of wetting properties. For example, cationic starch, natural starch, and synthetic polymers such as wet strength resins have been added to tissue and paper webs in general to modify dry strength, wet strength, water resistance, or other properties, or to improve certain manufacturing processes. In tissue making, such additives have also been used to improve the manufacturing process of creping. For example, cationic starch has been proposed as a creping aid to be applied in aqueous solution to the surface of a Yankee dryer prior to adhering an embryonic tissue web to the drum. Such a proposal is made by Vinson et al. in U.S. Pat. No. 6,207,734, “Creping Adhesive For Creping Tissue Paper,” issued on Mar. 27, 2001, which discloses creping of tissue with a creping additive comprising cationic starch having between about 0.001 and about 0.2 cationic substituents per anhydroglucose unit of the cationic starch. However, it is known that in typical cationic starch, cationic groups (quarternary ammonium) are added to starch with a degree of substitution of about 2 to 5% (from about 0.02 to about 0.05 cationic substituents per anhydroglucose unit of the cationic starch), resulting in about 0.15% to about 0.5% nitrogen by weight added to the cationic starch. This degree of cationic substitution in the starch is an inherent limitation of conventional cationic starch, in which the degree of cationic substitution is typically constrained to be less than about 0.1 in practice because of processing difficulties when higher degrees of substitution are attempted. Starch and cationic starch also pose practical difficulties in their application to a paper web due to limited water solubility and high viscosity. Typical cationic starch is provided as granules and has a molecular weight of about 50 to 500 million.
The limitations in chemistry and physical properties of cationic starch translate into limitations in what may be achieved when cationic starch is used in creping or in other papermaking processes.
The processes and chemistry used in creping and other tissue making or paper making processes may play a significant role in product quality, cost, and runnability. Improvements in the chemistry used to adhere tissue web to the surface of a Yankee dryer, for example, may improve the balance between the need to firmly adhere the tissue web to the surface of the Yankee dryer and the need to release the tissue web from the surface of the Yankee dryer. If the adhesion is too strong, the tissue web can be damaged upon creping. If too weak, the proper degree of softness and bulk may not be obtained by the mechanical action of the creping blade, or the contact between the tissue web and the dryer may be inadequate to promote good drying, or the tissue web may separate prematurely and break. If the creping aid is not applied uniformly due to processing limitations (poor mixing or poor spray performance due to high viscosity, or plugging of spray nozzles due to particulates or poor solubility), then nonuniform creping may occur. If too much of the creping aid remains on the tissue web or of the creping aid forms too thick of a layer on the surface of the Yankee dryer, then the tissue may have undesirable deposits or coloration or odor.
Creping is not the only situation in which a balance between adhesion and release is needed for a tissue product in contact with a surface. There is a need for improved control of adhesion and release of tissue from surfaces on which no creping is involved, such as from through air dryer fabrics and other drying fabrics, impression fabrics, drum dryers, embossing rolls, calendering rolls, rush transfer zones, and other nips and surfaces.